Boron

This article is about the chemical element. See Boron, Territoire de Belfort for the commune of the Territoire de Belfort département in France.

Boron is the chemical element in the periodic table that has the symbol B and atomic number 5. A trivalent metalloid element, boron occurs abundantly in the ore borax. There are two allotropess of boron; amorphous boron is a brown powder, but metallic boron is black. The metallic form is hard (9.3 on Mohs' scale) and a bad conductor in room temperatures. It is never found free in nature.

Notable characteristics

Boron is electron-deficient, possessing a vacant p-orbital. Compounds of boron often behave as Lewis acids, readily bonding with electron-rich substances in an attempt to quench boron's insatiable hunger for electrons.

Optical characteristics of this element include the transmittance of infrared light. At standard temperatures boron is a poor electrical conductor but is a good conductor at high temperatures.

Boron has the highest tensile strength of any known element.

Boron nitride can be used to make materials that are as hard as diamond. The nitride also acts as an electrical insulator but conducts heat similar to a metal. This element also has lubricating qualities that are similar to graphite. Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks.

Applications

The most economically important compound of boron is sodium tetraborate decahydrate Na₂B₄O₇ · 10H₂O, or borax, which is used in large amounts in making insulating fiberglass and sodium perborate bleach. Other uses:

Because of its distinctive green flame, amorphous boron is used in pyrotechnic flares.
Boric acid is an important compound used in textile products.
Compounds of boron are used extensively in organic synthesis and in the manufacture of borosilicate glasses.
Other compounds are used as wood preservatives, and are particularly attractive in this regard because they possess low toxicity.
Boron-10 is used to assist control of nuclear reactors, a shield against radiation and in neutron detection.
Boron filaments are high-strength, lightweight materials that are chiefly used for advanced aerospace structures.
Sodium borohydride (NaBH₄), is a popular chemical reducing agent, used (for example) for reducing aldehydes and ketones to alcohols.

Boron compounds are being investigated for use in a broad range of applications, including as components in sugar-permeable membranes, carbohydrate sensors and bioconjugates. Medicinal applications being investigated include boron neutron capture therapy and drug delivery. Other boron compounds show promise in treating arthritis.

Hydrides of boron are oxidized easily and liberate a considerable amount of energy. They have therefore been studied for use as possible rocket fuels.

History

Compounds of boron (Arabic Buraq from Persian Burah) have been known of for thousands of years. In early Egypt, mummification depended upon an ore known as natron, which contained borates as well as some other common salts. Borax glazes were used in China from 300 AD, and boron compounds were used in glassmaking in ancient Rome.

The element was not isolated until 1808 by Sir Humphry Davy, Joseph Louis Gay-Lussac, and Louis Jacques Thénard, to about 50 percent purity. These men did not recognize the substance as an element. It was Jöns Jakob Berzelius in 1824 who identified boron as an element. The first pure boron was produced by the American chemist W. Weintraub in 1909.

Occurrence

The United States and Turkey are the world's largest producers of boron. Boron does not appear in nature in elemental form but is found combined in borax, boric acid, colemanite, kernite, ulexite and borates. Boric acid is sometimes found in volcanic spring waters. Ulexite is a borate mineral that naturally has properties of fiber optics.

Economically important sources are from the ore rasorite (kernite) and tincal (borax ore) which are both found in the Mojave Desert of California (with borax being the most important source there). Turkey is another place where extensive borax deposits are found.

Pure elemental boron is not easy to prepare. The earliest methods used involve reduction of boric oxide with metals such as magnesium or aluminium. However the product is almost always contaminated with metal borides. (The reaction is quite spectacular though.) Pure boron can be prepared reducing volatile boron halogenides with hydrogen at high temperatures.

In 1997 crystalline boron (99% pure) cost about US$55 per gram and amorphous boron cost about US$2 per gram.

Isotopes

Boron has two naturally-occurring and stable isotopes, ¹¹B (80.1%) and ¹⁰B (19.9%). The mass difference results in a wide range of δB-11 values in natural waters, ranging from -16 to +59. There are 13 known isotopes of boron, the shortest-lived isotope is ⁷B which decays through proton emission and alpha decay. It has a half-life of 3.26500x10^-22 s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(O H)₃ and B(OH)₄. Boron isotopes are also fractionated during mineral crystallization, during H₂O phase changes in hydrothermal systems, and during hydrothermal alteration of rock. The latter effect species preferential removal of the ¹⁰B(OH)₄ ion onto clays results in solutions enriched in ¹¹B(OH)₃ may be responsible for the large ¹¹B enrichment in seawater relative to both oceanic crust and continental crust.

Precautions

Elemental boron and borates are not toxic and therefore do not require special precautions while handling. Some of the more exotic boron hydrogen compounds, however, are toxic and do require special handling care.